Induction logging antenna

ABSTRACT

An antenna assembly for an induction logging tool for use in a drilling operation. The assembly includes an outer pressure housing, a coil former within the pressure housing, magnetic cores mounted in the housing, a core carrier within the coil former and the pressure housing which carries the magnetic cores, and shock-absorbing mountings on the core carrier for the magnetic cores.

[0001] This invention relates to an antenna assembly for an inductionlogging tool.

[0002] The invention has been developed primarily, though notexclusively, with a view to provide an improved antenna assembly for aninduction logging tool of the general type disclosed in more detail inU.S. Pat. No. 6,100,696 and PCT/GB00/02510. However, the improvedantenna assembly of the invention is applicable generally to inductionlogging tools.

[0003] In designing an antenna assembly for an induction logging tool,consideration has to be given to the external environmental conditionswhich prevail during a typical drilling situation e.g. pressures of upto 20,000 p.s.i., temperatures ranging from −35° C. to +175° C., andshock and vibration to 1000 g. Despite the environmental factors it isimportant to maintain electrical stability, and in particular selfinductance of the tool in order to achieve accurate logging.

[0004] Conventional wisdom suggests that it is not possible to employmagnetic material in an induction logging tool, in a drillingenvironment, and while maintaining electrical stability of the antennaassembly. Typically, antenna instability in an induction logginginstrument results in diminished accuracy of the measurement.

[0005] In addition, typically antenna instability in an inductionlogging instrument results in diminished accuracy of the loggingmeasurement. Furthermore, MPP (molypermalloy)—used in logging tools, isa brittle material and is at risk of being damaged in a drillingenvironment.

[0006] The use of magnetic materials in a coil also effectively permitsa reduction in antenna dimensions, more particularly the area enclosedby the wound coil. In this case, an effective permeability of between5-6 (produced by the magnetic core) allows a reduction in coil diameterof around 2.5, for same antenna sensitivity.

[0007] The invention seeks to provide an improved antenna assembly whichcan provide suitable protection of the magnetic core under shock andvibration and axial thermal expansion of supporting materials i.e. tominimise stresses on the magnetic cores. This is an important objectivesince the permeability of the cores is a complex function which includesthe stresses observed by the material, and a change in the permeabilityof the material results in a change in the self inductance of theantenna. These stresses may arise CONFIRMATION COPY from several sourcese.g. hydrostatic pressure, thermal expansion of supporting structuresand shock and vibration.

[0008] According to the invention there is provided an antenna assemblyfor an induction logging tool for use in a drilling operation, saidassembly comprising:

[0009] an outer pressure housing;

[0010] a coil former within the pressure housing;

[0011] magnetic cores mounted in the housing;

[0012] a core carrier within the coil former and pressure housing andwhich carries the magnetic cores; and

[0013] shock-absorbing mountings on the core carrier for the magneticcores.

[0014] Therefore, the invention provides an antenna assembly for aninduction logging tool which maintains its electrical integrity despiteadverse environmental conditions, and in particular which is resistantto adverse local pressure, temperature and shock loadings.

[0015] The configuration of the antenna assembly serves to reducestresses induced by local environmental conditions and their effect onother significant antenna variables, such as effective antennaaperture/area, and self inductance.

[0016] Preferably, the magnetic cores are MPP cores and which,advantageously, undergo special processing prior to final assembly. Thepurpose of the processes is threefold: to optimise the magnetic couplingbetween the individual cores (and hence optimise effectivepermeability), and to relieve post machining stresses, and reduce localstresses on the magnetic core surface when held in compression,maintaining stability. The process involves machining and “lapping” thetoroidal cores to maximise the magnetic coupling area between them andproduce an accurate surface finish at the interface between the cores,followed by a temperature cycle annealing process to reduce stressesbuilt up during the machining process.

[0017] A preferred embodiment of an antenna assembly according to theinvention will now be described in detail, by way of example only, withreference to the accompanying drawings, in which:

[0018]FIG. 1 is a sectional elevation view through the antenna assembly,on a plane indicated by the line C-C on FIG. 4;

[0019]FIG. 2 is a partial sectional elevation view through the antennaassembly on the same plane as FIG. 1;

[0020]FIG. 3 is again, a partial sectional elevation view, to anenlarged scale, through the antenna assembly on the same plane as FIG.1, but showing greater detail of some of the component parts;

[0021]FIG. 4 is a cross sectional view of the assembly on a planeindicated by B-B in FIG. 1;

[0022]FIG. 5 is a cross sectional view of the assembly on a planeindicated by A-A in FIG. 1, and showing a first arrangement of shockabsorbing mountings for magnetic cores of the antenna assembly;

[0023]FIG. 6 is a cross sectional view of an alternative arrangement ofshock absorbing mountings; and,

[0024]FIGS. 7 and 8 correspond to FIGS. 5 and 6 respectively, and show,to an enlarged scale, the differences in the internal arrangement of theinnermost of the shock absorbing mountings between the radially outersurface of a core former and the radially inner surface of the magneticcores.

[0025] Referring now to FIGS. 1 to 4 of the drawings, there is shown anantenna assembly for an induction logging tool (for use in drillingoperations) and which has been designed to maintain its electricalintegrity (and hence the accuracy of the logging tool), despite theadverse environmental factors which prevail in a typical drillingsituation.

[0026] The antenna assembly is designated generally by reference 20, andhas been designed specifically for use with an induction logging tool ofthe general type disclosed in U.S. Pat. No. 6,100,696 or PCT/GB00/02510.

[0027] The assembly has an outer pressure housing 3, and toroidalmagnetic cores 6 of MPP (molypermalloy) which are mounted inshock-resistance manner in the housing. Shock-absorbing mountings 7mount the cores 6 in the housing 3, and are made of VITON or NITRILE(Trade Mark) elastomer.

[0028] Further components of the assembly 10 include a coil former 1made of cylindrical epoxy glass fibre tube, and core spacers 2 which arethick-walled cylindrical epoxy glass fibre tube. The outer pressurehousing 3 also is made of cylindrical epoxy glass fibre tube. A coreformer 4 is also made of cylindrical glass fibre tube, and reference 5denotes a super shield made of a ferrous metal core with a coppersheath. The glass fibre tube is preferably of type with low coefficientof thermal expansion, in the axial direction, and capable ofwithstanding temperatures up to 175° C.

[0029] O-rings 8 are made of VITON or NITRILE elastomer, and reference 9denotes Litz wire which is a high temperature insulated multi-strandwire. 10 is adhesive tape (KAPTON tape), and of polyimide, and 11 is anelectrostatic shield manufactured from flexible printed circuit(polyimide and copper). The Kapton tape (polyimide film) restrictsunwanted movement or radial thermal expansion of the Litz wire woundaround the coil former, maintaining the dimensional stability of thecoil and hence its effective aperture and self inductance. 12 ishydraulic oil (Shell Tellus 15), and 13 are copper conductors.

[0030] As can be seen in FIG. 1, the shock absorbing mountings 7comprise three different types of mounting blocks, namely an axiallyshort outer sleeve 7 a, a longer inner sleeve 7 b, and a pair of annularcollars 7 c arranged one at each end of a stack of axially spacedmagnetic cores 6.

[0031]FIGS. 5 and 7 show cross sections of the arrangement of mountings7 in more detail, and in particular of the inner sleeve 7 b on the coreformer 4 with a narrow axial slot 14 providing a path for hydraulic oil12.

[0032] An alternative arrangement of the mountings 7 is shown in FIGS. 6and 8, in a preferred development of the invention. In particular, theinnermost shock absorbing mountings (7 b) of the first arrangement (seeFIGS. 5 and 7) are modified. The inner sleeve 7 b of FIGS. 5 and 7 is acontinuous layer of VITON which is wrapped around the core former 4,below the magnetic cores 6, and having a small oil path 14. This isreplaced in the arrangement of FIGS. 6 and 8 by three axially extendingshock absorbing blocks in the form of three separate dampers 15 whichare of circular cross section, and which are located in axial slotsmachined in the outer surface of the core former 4, and which run thefull length of the core assembly. The slots are positioned at 120° apartspacing circumferentially of the core former 4. In addition, three smalllateral holes between the internal surface of the core former 4 and thecavity created between the inner surface of the magnetic cores 6 and theouter surface of the core former 4 are drilled to provide hydrauliccoupling, one such hole being shown by reference 16 in FIG. 8, toprovide an oil path.

[0033] This preferred development has the following additionaladvantages, over and above the first described arrangement of mountingblocks. These advantages are as follows:

[0034] 1. Thermal expansion of the shock absorbing materials beneath themagnetic cores 6 no longer applies stresses to the magnetic material;

[0035] 2. Hydraulic balance between the outside and inside surfaces ofthe magnetic cores 6 can be guaranteed, again minimising stressesapplied to the magnetic materials;

[0036] 3. It is envisaged that lateral damping from shock and vibrationwill also be considerably improved.

1. An antenna assembly (20) for an induction logging tool for use in adrilling operation, said assembly comprising: an outer pressure housing(3); a coil former (1) within the pressure housing (3); magnetic cores(6) mounted in the housing (3); a core carrier (4, 5) within the coilformer (1) and the pressure housing (3), and which carries the magneticcores (6); and, shock-absorbing mountings (7) on the core carrier (4, 5)for the magnetic cores (6).
 2. An antenna assembly according to claim 1,in which the shock-absorbing mountings (7) include a sleeve (7 b)mounted on the core carrier (4, 5).
 3. An antenna assembly according toclaim 1, in which the shock-absorbing mountings (7) include a number ofseparate axially extending elongate blocks (15) circumferentially spacedfrom each other around the outer surface of the core carrier (4, 5),such blocks being located between the outer surface of the core carrier(4, 5) and the inner surface of the magnetic cores (6).
 4. An antennaassembly according to any one of claims 1 to 3, in which the shockabsorbing mountings (7) include an outer sleeve (7 a) located betweenthe outer surfaces of at least some of the magnetic cores (6) and theinner surface of the pressure housing (3).
 5. An antenna assemblyaccording to any one of claims 1 to 4, in which the shock absorbingmountings (7) include annular collars (7 b) arranged one at each end ofa stack of axially spaced magnetic cores (6).
 6. An antenna assemblyaccording to any one of the preceding claims, in which the core carriercomprises an inner metal core (5) and an outer cylindrical tube (4). 7.An antenna assembly according to any one of the preceding claims, inwhich the magnetic cores are annular cores (6) arranged in an axiallyspaced stack of cores mounted on the core carrier (4, 5).
 8. An antennaassembly according to any one of the preceding claims, including ductsor holes (14, 16) which route hydraulic oil (12) to contact the magneticcores (6).